“Groot is a Flora Colossus from Planet X, the capital of the branch worlds. The Flora Colossi are tree-like beings whose language is almost impossible to understand due to the stiffness of their larynxes, causing their speech to sound like they are repeating the phrase “I am Groot”.” More on Groot from Wikipedia.

In the recent and hugely popular action movie Guardians of the Galaxy, Groot is a woody superhero played by Vin Diesel, able to lose limbs defending his human friends, seemingly without too much loss of power.